Interesting OSS project: Holesail creates instant P2P tunnels between networks (like a VPN)
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Reverse tunneling software | Holesail
https://holesail.io/
Holesail is the P2P company, building a truly peer to peer and open source reverse tunnelling software
"Create P2P tunnels instantly that bypass any network, firewall, NAT restrictions and expose your local network to the internet securely, no Dynamic DNS required."